Critical to the success of future multimedia services is the ability to provide fast access to stored information via communications networks. In the Nemesis project, we focus on application control protocols for delivering stored multimedia to a user. We are exploring adaptive rate control schemes that make use of predictive prefetch of information from remote storage servers as a strategy for coping with short-term network congestion. For users with low-end equipment or slow network connections, these protocols provide a degraded but still useful service. We also provide “better than being there” features such as variable playback rates with intelligible audio, and synchronization and linkage of multiple media.

HPC/VORX is a computing system that provides closely coupled computing between large numbers of processors. It also supports the connection of many host workstations which may be geographically distributed within the area of a large building and allows a single applications to span many processors and many workstations. We relate some of the lessons that were learned while building and using HPC/VORX and in the transition to HPC/VORX from a smaller, less capable system. The problems that we encountered included difficulties in scaling resource managers and human interfaces to large numbers of processors, the design of communications primitives and protocols, and the implementation of programming abstractions.

Solutions are presented for both complete and incomplete hypercube initialization problems. Current hypercube computers require that the node number and link numbers be hard-wired into each processor and link. The author introduces algorithms for determining node and link numbers on the fly whenever the system is power up. This makes it possible to exchange or replace nodes without the need of manually setting the node number or attaching interconnect cables to particular hardware ports. One assumption made by the algorithms is that each node has a unique identifier, called the processor identifier, that is used to resolve conflicts introduced by symmetry. It is shown to be efficient for each node to know its own link count to determine a numbering. By communicating among themselves, the nodes can determine the total number of nodes in the cube.>

Since a k-dimensional hypercube has 2/sup k/ vertices, these systems are restricted to having exactly 2/sup k/ computing nodes. Because system sizes must be a power of two, there are large gaps in the sizes of systems that can be built with hypercubes. Routing and broadcast algorithms are presented for hypercubes that are missing certain of their nodes, called incomplete hypercubes. Unlike hypercubes, incomplete hypercubes can be used to interconnect systems with any number of processors. The routing and broadcast algorithms for incomplete hypercubes are shown also to be simple and deadlock-free.>

Meglos is a multiprocessor operating system that is ideal for real-time applications. It provides the user with a distributed programming environment for developing, running, and debugging application programs. Meglos allows a single application to span several processors. Further, unrelated programs may run independently of each other on the same processor. A simple, yet powerful, mechanism for communications between programs is provided. With the current hardware configuration, up to twelve Motorola 68000 Multibus computer systems and a DEC VAX host running the UNIX operating system can be connected via the S/NET interconnect. Meglos exhibits communications latencies of 750 μsec, sufficiently small for most real-time applications.

Abstract Meglos provides a user‐level, message‐based programming environment for a system of interconnected processors. It allows the simultaneous execution of a wide range of applications including real‐time control and large‐scale computations requiring the co‐operative work of many processors. The communications mechanisms in Meglos are designed to be easy to use and to provide high performance at the applications level. The communications primitives are based on symmetric, error‐resilient channels that are independently flow controlled. Two‐ended channels between pairs of co‐operating processes provide communications with low latency and high throughput. Multicast channels are used to send messages efficiently among a large group of processors in a single operation.

A classical problem in concurrent program control is to provide a mechanism whereby several processes running concurrently can gain exclusive control of a resource. For each process, the section of its program in which it accesses the resource is called its critical section, and the problem is called the critical section problem. A solution to the critical section problem guarantees that no more than one process can be in its critical section at any time. The solution presented here improves on previous solutions by allowing processes to enter their critical sections on a first-come first-served basis.

Abstract Intuitively, the program size complexity of a binary string measures the amount of information in the string. Researchers have formalized this notion in a number of different ways. Here, we demonstrate similarities between some of these formulations. We also investigate in some detail the properties of Kolmogorov's complexity measure.